1. Field of the Invention
The present invention relates in general to a control apparatus for a vehicular power transmitting system, and more particularly to a control apparatus for a vehicular power transmitting system including an electrically controlled differential portion and a transmission portion, which control apparatus permits adequate shifting of the transmission portion so as to provide an optimum speed ratio.
2. Discussion of Prior Art
There is known a vehicular power transmitting system including (a) an electrically controlled differential portion which includes a differential mechanism and an electric motor connected to a rotary element of the differential mechanism and which is operable to control a differential state between rotating speeds of its input and output shafts by controlling an operating state of the electric motor, and (b) a transmission portion which constitutes a part of a power transmitting path and which functions as a step-variable automatic transmission portion. JP-2005-264762A discloses an example of such a vehicular power transmitting system. In the vehicular power transmitting system as disclosed in this publication, there are various combinations of a speed ratio of the electrically controlled differential portion determined by its differential state, and a speed ratio of the automatic transmission portion. In this respect, it is considered possible to improve the fuel economy of the vehicle, by suitably selecting the speed ratio of the electrically controlled differential portion.
For effecting shifting actions of the power transmitting system so as to maximize the fuel economy of a hybrid vehicle, it is theoretically desired to implement shift-up and shift-down actions in running states of the vehicle, which assure high power transmitting efficiency of the vehicle. If the shift-up action and the shift-down action between the same two gear positions are implemented in the same vehicle running condition, that is, if shift-up and shift-down boundary lines defined by a shifting boundary line map are identical with each other, namely, completely overlap each other, so-called “busy shifting actions” tend to take place, giving rise to a problem of deterioration of drivability of the vehicle. For instance, a shift-down action takes place immediately after a shift-up action. In view of this drawback, a suitable hysteresis is provided between the shift-up and shift-down boundary lines for shifting between the same two gear positions. The vehicle condition used for determining whether a shifting action should take place is represented by an opening angle of a throttle valve and a running speed of the vehicle, for example. In this case, the hysteresis is provided such that the shift-down boundary line is located on a high vehicle-drive-force side of the shift-up boundary line, so that the shift-down action is implemented at a lower vehicle running speed or at a larger operation amount of an accelerator pedal (at a larger opening angle of the throttle valve), than when the shift-up action is implemented.
It is noted that the power transmitting efficiency of the vehicle is calculated on the basis of at least the operating speed of a vehicle drive power source such as an engine and the output shaft speed of the power transmitting system. On the other hand, the vehicle condition used for determining the shifting actions is generally represented by the vehicle running speed or any other parameter corresponding or proportional to the vehicle running speed, and the operating amount of the accelerator pedal or any other parameter indicative of the required vehicle drive force. While the vehicle is running in a steady state or in a slowly accelerating or decelerating state, the operating speed of the vehicle drive power source follows the operation amount of the accelerator pedal with a relatively high response, so that the shifting actions of the power transmitting system can be implemented on the basis of the operation amount of the accelerator pedal, with a high degree of the power transmitting efficiency which depends on the operating speed of the vehicle drive power source.
When the accelerator pedal is abruptly depressed, however, the power transmitting system undergoes a shift-down action called “a kick-down”. In this case, a response of a change of the operating speed of the vehicle drive power source to an abrupt change of the operation amount of the accelerator pedal is lower than that in the steady running state or slowly accelerating or decelerating state of the vehicle. That is, a rise of the operating speed of the vehicle drive power source is considerably delayed with respect to an increase of the operating amount of the accelerator pedal, so that the operating amount of the accelerator pedal and the operating speed of the vehicle drive power source are not closely related to each other. Therefore, if a shifting action is commanded to take place on the basis of the operating amount of the accelerator pedal related to a target operating speed of the vehicle drive power source, the operating speed of the drive power source at which the shifting action takes place on the basis of the actual operation amount of the accelerator pedal is different from the target value. Accordingly, the shifting action is not implemented in the vehicle running state that permits a high degree of power transmitting efficiency. This problem is not taken into account in the setting of the above-described hysteresis provided for preventing the so-called “busy shifting actions”.